1. Field of the Invention
The present invention relates to systems for achieving positional control of moveable elements without an absolute position feedback system. More particularly, the invention relates to a system for precisely controlling the positioning of a bleed valve in a gas turbine engine without the usage of an absolute position feedback system.
2. Description of the Prior Art
Gas turbine engines are in use which provide a variable quantity of compressed air from the engine compressor by a movable bleed valve and an output shaft which provides a power takeoff. The total power which can be drawn by the compressed air ducted by the bleed valve and the output shaft can exceed the rated horsepower of the engine. In order to avoid the possibility of operating gas turbine engines of the aforementioned type at power levels exceeding the rated horsepower of the engine, bleed valve control systems have been developed which reduce the amount of air being ducted by the bleed valve until the demand for compressed air and shaft output power is not greater than the rated power of the engine. These bleed valve control systems monitor the actual exhaust gas temperature and reduce the amount of air being ducted so that the actual exhaust gas temperature does not exceed the rated maximum exhaust gas temperature of the engine thereby insuring that the output power does not exceed the rated power.
FIG. 1 illustrates a block diagram of a prior art system for controlling a bleed valve in a gas turbine. The system 10 controls the positioning of a bleed valve of the bleed valve and gas turbine 12 to ensure that the maximum rated output horsepower of the gas turbine is not exceeded by the combined horsepower drawn by the positioning of the bleed valve to duct compressed air and the shaft output. During operation, when the total horsepower being drawn by ducting of compressed air and output shaft horsepower varies, the positioning of the bleed valve is controlled so that when the sensed exhaust gas temperature exceeds a maximum exhaust gas temperature (M.E.G.T.), the bleed valve is closed so that the amount of compressed air ducted by the bleed valve is reduced until the actual exhaust gas temperature (E.G.T.) is equal to or less than the M.E.G.T. A valve positioning command, which is equal to the difference between the M.E.G.T. and the E.G.T., is applied to a differential amplifier 14 which produces an output signal proportional to the difference between the valve positioning command and an absolute position signal outputted by position feedback system 16. The position feedback system 16, which may be any known position feedback system, such as a synchro, potentiometer, mechanical or optical encoder or proportional optical device, provides an output signal directly proportional to the absolute position of the bleed valve. The output signal from the proportional amplifier 14, which may be differential amplifier, is applied to a pulse width control circuit 18 which produces a series of output pulses having a width which varies in direct proportion to the output signal and is synchronized with the output frequency of clock 20. The pulse width control may be implemented by an absolute value amplifier which produces an output signal directly proportional to the absolute value of the output signal from differential amplifier 14. The output of the absolute value amplifier is applied to a comparator which compares the magnitude of the absolute value output signal with a ramp signal synchronized at the basic clock frequency which rises from a zero level at the initiation of each cycle of the clock. The comparator produces an output signal having a duration from the beginning of the clock's cycle until the point at which the value of the ramp signal exceeds the magnitude of the output signal from the absolute value. A pair of flip-flops 22 and 24, respectively, control the opening and closing of the bleed valve. The flip-flops 22 and 24 may be implemented as D-type flip/flops of known construction. The output signal from the differential amplifier 14 is also coupled to a direction control 26 which is implemented as a zero crossing detector and inverter to produce output signals Q and Q which respectively are the data inputs for flip-flops 22 and 24. The output signal Q is high when the polarity of the output signal from the differential amplifier 14 has a first sign (positive or negative) and is low when the polarity of the output signal has the other sign. The output signal Q has a bistable level which is opposite that of the signal Q. As long as the data signal to either of the flip-flops 22 or 24 is high, the output from that flip-flop has a pulse duration equal to the duration of the pulse produced by the pulse width control 18. Accordingly, the width of the pulses for opening or closing the bleed valve are directly proportional to the pulse width of the pulses outputted by the pulse width control 18. As soon as the data signal goes low to either of the flip-flops 22 or 24, signifying that a change in direction of the movement of the bleed valve 12 should occur, the output of the flip-flop 22 or 24 which was high goes low until an opposite change in direction occurs at a later time.
While the operational control produced by this system is satisfactory in achieving an overall control characteristic for the gas turbine such that the total of the power outputted from the engine in the form of shaft horsepower and compressed air does not exceed the maximum rated power for the gas turbine engine, it suffers from the disadvantage of requiring expensive position feedback controls in its implementation.